How can I edit F-curves of an action already put in the NLA editor
Position your cursor over the track in the NLA editor and hit TAB.  This will enable the editing mode for that NLA track (it should turn green).  If you have an F-curve editor open at the same time, the animation curve for the NLA track will appear in it a allow editing.  (Ensure that the 'Only include channels relating to selected objects and data' option is NOT selected in the F-curve toolbar).